Conventionally, as a method for measuring depth of a hardening layer formed on a surface of a member of a steel material by hardening process (hardening depth), there is known a method that a part of members hardened at the same batch is cut and texture observation of the cut surface is performed or distribution of Vickers hardness in the depth direction of the cut surface is measured.
However, the method has various problems such as shown below. (1) The process of cutting a part of members which is possible to be a product is included so that the cut measurement object must be scraped, whereby yield of the productions is reduced. (2) A series of processes such as cutting, processing of the cut surface (grinding, etching and the like), observing of the cut surface with an electron microscope or measuring hardness with a Vickers hardness testing machine so that time required for the measurement is long. (3) According to the above-mentioned reasons, the method is not applicable to total inspection. (4) The total quality assurance by the sampling inspection has a limit, and according to the measurement result of the measurement object, all the members hardened at the same batch must be treated as nonconforming articles, whereby yield of the productions is also reduced.
For solving such the problems, noncontact measurement of the hardening depth with a so-called eddy current sensor is examined. Examples thereof are described in the Patent Literatures 1 and 2.
In the method described in the Patent Literature 1, a magnetization coil of the eddy current sensor inserted thereto with a measurement object generates an alternating current magnetic field, eddy current is generated on the surface of the measurement object by the alternating current magnetic field, magnitude of an induced magnetic field generated by the eddy current is detected as output voltage with a detection coil of the eddy current sensor inserted thereto with the measurement object, and the relation between hardening depth of a known measurement object of the same material with output voltage is compared with the output voltage of the detection coil so as to measure depth of a hardening layer.
In the method described in the Patent Literature 2, alternating current voltage (alternating current magnetization signal) of a plurality of different frequencies is applied on a magnetization coil of the eddy current sensor inserted thereto with a measurement object, eddy current is generated on the surface of the measurement object by the magnetization coil, magnitude of an induced magnetic field generated by the eddy current is detected as output voltage (detection signal) with a detection coil of the eddy current sensor inserted thereto with the measurement object, distribution of hardness in the depth direction of the measurement object is measured based on amplitude ratio of the alternating current magnetization signal and the detection signal, and hardening depth of the measurement object is measured based on phase difference of the detection signal about the alternating current magnetization signal.
According to the method described in the Patent Literature 2, the distribution of hardness in the depth direction and the hardening depth of the measurement object can be measured noncontactly simultaneously, and the method is applicable to the total inspection.
However, the methods described in the Patent Literatures 1 and 2 have a problem that, when the temperature of the measurement object at the time of measurement is changed by rot change of the measurement object or change of measurement environment, the measurement result of the hardening depth is changed, whereby accurate measurement of the hardening depth is difficult.
That is because the change of the temperature of the measurement object causes change of permeability and conductivity of the measurement object, whereby the output voltage (detection signal) of the detection coil is changed.
In the case that the measurement object is a drive shaft made by a steel material hardened with high frequency, environment temperature of a production factory of the drive shaft is changed for about 30° C. between the daytime of midsummer (about 35° C.) and the early morning of midwinter (about 5° C.), and the temperature is changed widely between the daytime and nighttime of the same day.
Since the temperature of the drive shaft is higher than the environment temperature, in the case that the time from the hardening to the start of measurement of the hardening depth is different from that at ordinary times, for example the case that the production factory stopped temporary is restarted, the temperature of the drive shaft at the time of measurement of the hardening depth is changed.
As a method for preventing the reduction of measurement accuracy of the hardening depth caused by such temperature change, below methods are supposable. (1) The measurement is performed while temperature of the measurement object, the measurement device and the environment around them is kept constantly by air conditioning equipment. (2) The temperature of the measurement object is measured just before the measurement of the hardening depth and the measurement result of the hardening depth is corrected based on the measured temperature.
However, in the method (1), the cost of equipment is large, and the measurement cannot be performed until the temperature of the measurement object, the measurement device and the environment around them are kept constantly (the working efficiency is not good).
In the method (2), the temperature measurement of the measurement object is performed in addition to the measurement of the hardening depth so that the number of the processes is increased. Especially, since the temperature measurement is difficult to be performed for short time generally (the object must be kept until the measurement temperature reaches equilibrium condition), time for the measurement of the hardening depth of one measurement object is long, whereby it is difficult to apply the method to the total inspection in the production process of the measurement object.
For solving the above problems, the inventor has obtained below knowledge. The alternating current magnetization signals of two different frequencies are applied on a drive shaft with a magnetization coil so that induced currents of eddy currents respectively corresponding to the two different frequencies are generated in the drive shaft. Detection signals caused by the induced currents corresponding to the two different frequencies are detected with a detection coil. Based on amplitude of the detection signal corresponding to one of the two different frequencies, phase difference between the alternating current magnetization signal and the detection signal corresponding to one of the two different frequencies, amplitude of the detection signal corresponding to the other of the two different frequencies, and phase difference between the alternating current magnetization signal and the detection signal corresponding to the other of the two different frequencies, difference D shown in below formula 1 is calculated. The hardening depth of the drive shaft is measured based on the difference D. According to the method of performing the above-mentioned series of works, the hardening depth of the drive shaft can be measured noncontactly (nondestructively) quickly regardless of the change of the environment temperature and the temperature of the drive shaft itself, whereby the method is applicable to the total inspection of the measurement object (in-line inspection).
                    D        =                                                                              X                  1                  2                                +                                  Y                  1                  2                                                      -                                                            X                  2                  2                                +                                  Y                  2                  2                                                                                                        X                2                2                            +                              Y                2                2                                                                        [                  Formula          ⁢                                          ⁢          1                ]            
However, the above method has below problems.
For securing enough measurement accuracy in the method, it is necessary to select appropriately the combination of two different frequencies of the alternating current magnetization signals applied on the drive shaft. However, the combination of two different frequencies must be selected by trial and error (concretely, detection signals are obtained while the combination of frequencies is changed variously about a plurality of measurement portions of the measurement object, and based on the measurement, the result of calculation of hardening depth is compared with hardening depth actually obtained by cutting the measurement portion and inspecting with a microscope or measuring Vickers hardness), whereby much man-power and time is required for selecting the appropriate combination of two frequencies.
Since the appropriate combination of two frequencies is changed following the change of shape or measurement portion of the measurement object, the work for selecting the appropriate combination of two frequencies must be performed for every measurement object or every measurement portion.
The cause of the trial and error in the selection of the combination of frequencies is decrease of measurement accuracy following change of the environment temperature and the temperature of the measurement object. Then, the method for selecting the combination of frequencies is desirable that the mechanism of temperature dependency of the measurement accuracy is elucidated and the temperature dependency of the measurement accuracy is reduced or eliminated efficiently in accordance with a fixed rule.    [Patent Literature 1] JP 2002-14081 A    [Patent Literature 2] JP 2004-108873 A